1. Field of the Invention
The present invention relates to an optical reader for optically reading an information pattern representing coded data and, more specifically, to an optical reader suitable for reading a two-dimensional information pattern representing coded data.
2. Description of the Related Art
Recently, a tessellated two-dimensional information pattern representing coded data, in which black zones and white zones are two-dimensionally arranged to represent coded data, has been proposed and, further, an optical reader for reading and decoding the coded data from the tessellated information pattern has been developed. The optical reader is frequently constituted as a hand held-type reader, comprising a reading head including a hollow box-like hand casing which has an opening formed in a bottom wall, thereby defining a reading area. The reading head also includes a two-dimensional CCD (charge coupled device) image sensor associated with an image formation lens, which is assembled in a hole formed in a top wall of the head casing. The reading head further includes a light source, incorporated in the top wall of the head casing, which illuminates the reading area defined by the opening of the head casing.
For example, when a two-dimensional information pattern is recorded on an article, the optical reader is placed relative to the article such that the information pattern is encompassed by the reading area of the reading head thereof. Then, the information pattern is illuminated by the light source, and an image of the pattern is focused on a light receiving surface of the image sensor by the image formation lens. The focused image is thereby converted into image signals by the image sensor. The image signals, having been read out from the image sensor, are processed and converted into digital data. Further processing is then required to enable The digital data to be decoded. Once the decoding process is completed, the optical reader produces a visual indication or audio indication, whereby a user can recognize that the optical reading of the information pattern concerned has been properly carried out. Note, if necessary, the decoded data is transmitted to, for example, a host computer which is in communication with the optical reader.
Before the proper optical reading of the two-dimensional information pattern by the image sensor can be completed, the reading head of the optical reader must be properly positioned, with respect to the article, such that the information pattern is completely encompassed by the reading area of the recording head.
However, even if the reading head is properly aligned, the reading head may not be sufficiently stationary, for example, the hand of the user may be trembling. Alternatively, the reading head may be positioned in such a manner that the distance between the article, on which the two dimensional information pattern is recorded, and the reading area is not kept consistent for every portion of the reading area. Accordingly, the image of the two dimensional information pattern is not properly focused on the light receiving surface of the image sensor and, therefore, the optical reading of the pattern is not completed.